1These tags are discouraged, for stylistic reasons.
And <pre> should be eschewed in favor of <code> (see next).

2<code> and <c>,
used for displaying code/data,
are not true HTML tags, but are interpreted by the PerlMonks engine.
They inhibit the normal interpretation of enclosed HTML special characters like <, >, &, [, and ].
Any newlines in the enclosed code will be rendered such that long lines wrap.
Like HTML, <code> and <c> tags are case ignored.
Unlike HTML, whitespace is not allowed inside (e.g., < code > will not be recognized),
and they must have a matching closing tag (</code> or </c>) for them to work.
It is not possible to enclose </code> within a <code> block, nor </c> in a <c> block; they would be taken as block closers instead.

3readmore,
used for abbreviating very long posts within the context of other posts,
is not a true HTML tag, but is interpreted by the PerlMonks engine.
A readmore block is rendered as a <div class="readmore"> block when viewing the node directly, and as a "Read more..." placeholder link otherwise.
The link is to a page where you are viewing the node directly, thus displaying the full content.
Unlike HTML, whitespace is not allowed inside (e.g., < readmore > will not be recognized).
If a matching closing tag (</readmore>) is not given, the readmore block is taken to extend to the end of the body of your post.

4spoiler,
used for hiding spoilers,
is not a true HTML tag, but is interpreted by the PerlMonks engine.
Each reader can choose how spoiler blocks will be rendered.

5The <wbr> HTML element is used to indicate a suggested word-break point for wrapping long strings with no whitespace.
See documentation at the W3C.